During the manufacture of printed circuit cards, a clean metal surface or substrate is coated with a photo resist. A mask having the desired pattern is then placed over the resist and exposed to light of an appropriate wavelength which hardens the resist in the exposed areas. The resist film is then washed to disolve the unexposed resist and thereby uncover part of the metal substrate. The board may now be further treated to render the exposed photo resist etch resistant, after which the uncoated substrate is etched away and the remaining resist can be removed. In the continued effort to decrease the bulk of electronic equipment, there has been a trend to produce boards with a higher density of components. This tendency has brought about a consequent greater density of desired circuit lines that must be created on the board. As a consequence the circuit lines are placed closer together. This closeness of the lines on the board has been followed by a number of production problems. Various defects can occur during the exposure operation, including thermal expansion of the pattern and lack of exposure of the resist due to unwanted opaque particles close to the pattern blocking radiation. Small opaque particles may cling to the mask and be reproduced on the final circuit board as an undesired open connection of a number of the lines.